A shock absorber that is previously well known from document 1 (Japanese Unexamined Utility Model Registration Application Publication No. S62-194946) or the like is provided with a cylinder chamber in a tube, the cylinder chamber enclosing a liquid therein, a piston dividing the cylinder chamber into two chambers, and a piston rod, one end of which is connected to the piston and the other end of which extends out from the tube, and allows the piston rod to generate a damping force by controlling an outflow of a viscous liquid from the compression side of the cylinder chamber due to sliding of the piston in the cylinder chamber.
What is described in the document 1 as a prior art is such that, as a shock absorber that controls an outflow of a liquid from the compression side of a cylinder chamber due to movement of a piston, a cylinder sealed at one end is constructed to be double-walled, a plurality of through holes disposed on the inner cylinder wall surface at a predetermined pitch in a direction of the cylinder axis are formed, and a viscous fluid, which is precluded when the piston moves inside the inner cylinder in its axis direction, is allowed to flow outside the inner cylinder through the through holes.
Then, what is described in the document 1 as the invention is such that an inner tube is disposed inside an outer tube with a predetermined clearance, a viscous liquid is enclosed in a liquid chamber between both the tubes and a cylinder chamber in the inner tube, a throttle valve is installed in a connecting passage for connecting the liquid chamber and the cylinder chamber, and further a cross-sectional area of the flow passage between an external periphery of a piston and the internal periphery of the inner tube becomes gradually smaller toward the direction in which a piston rod is pushed in, by making an internal periphery of the inner tube tapered so that its inner diameter is gradually decreased toward the direction in which the piston rod is pushed in.
However, this type of shock absorber, that is capable of controlling an amount of liquid flowing out in dependence of a piston stroke, has a configuration such that a liquid discharged from the compression side of the cylinder chamber, when the piston rod is moved toward the direction of being pushed in due to a mechanical impact applied to the piston rod, is transferred to between the inner tube and the outer tube; thereby the cylinder tube must consequentially be constructed to be double-walled, which results in a problem that the number of parts becomes large and the structure becomes complicated. Moreover, when a tapered portion is provided on the internal periphery of an inner tube, an accuracy of forming a tapered surface has a great effect on the cross-sectional area of a flow passage defined by the clearance between an external periphery of a piston and the internal periphery of the inner tube, so there was a problem not only that high accuracy manufacturing of the tapered surface is required, but also that it was difficult to accurately control a damping force being applied to the piston rod.